Some MODFLOW River Modelling Issues

Posted for Tom Maddock


1) A Reach is that portion of the river or stream that that is associated with a particular finite-difference cell used by the groundwater flow model.

· 2)A Segment is a group of reaches that have 1) uniform or linearly changing hydraulic properties such as streambed elevation, streambed thickness, streambed hydraulic conductivity, and stream depth and width; 2) tributary flows or specified inflows or outflows (at boundaries); 3) diversions.

· 3) The numbering of segments and reaches is important. Segments are numbered sequentially from the furthest upstream to the last downstream segment.

· 4) Reaches within a segment must be numbered sequential from the furthest upstream reach in a segment to the last downstream reach of the segment.

· 5) The length of river within a cell is a function only of the cell size. Cell size is a precision issue.

· 6) Stream depth is used to compute the head in each reach by adding stream depth to the top of the streambed elevation for each reach. Stream depth is computed at the midpoint of each reach.

7) There are five options for computing stream depth: 1) Specify the stream depth for the first and last reach of a segment, and the program then linearly interpolates over the other reaches within the segment; 2) Uses Manning’s equation to determine depth as a function of flow,

Q = (C/N)A R^(2/3) So^(1/2)

where Q is stream discharge, C is a constant depending on length units, n is Manning’s roughness coefficient, A is the cross-sectional area, R is the hydraulic radius, and S₀ is the slope of the stream channel; 3) also uses Manning’s equation but the channel geometry is divided into an eight point cross-sectional; 4) uses depth-discharge and width-discharge power function relations for estimating stream depth and width as function of flow,

y = cQ^f and w = aQ^b

where y is the depth, Q is streamflow, w is width and variables a, b, c, f are numerical coefficients determined by regression; 5) uses values of depth and width for given streamflows on the basis of observations at a streamflow gaging point (the data is entered in tabular form).

· A different option can be used for each segment.